Unless you're an electrical engineer, I doubt you're going to be able to do that. Plus, from my limited knowledge of circuits (I'm a Nuke student), it wouldn't even work that way because if one failed, the other one wouldn't work. They'd have to be hooked up in parallel and both plugged into the motherboard at the same time.
You could always buy another PSU and keep it in the closet after you test to make sure it works. Just keep it in an anti static bag so dust doesn't build up inside it or anything.